I have 2008 ford f350 (6.4 diesel). I have squeeling sound when I start my truck in the morning. As the truck gets warmer it somewhat goes away. I changed the idler pully, both tensioner pullys, but I believe there is one more. Why do I have this squeeling sound. It does sound like a bearing or pully. Please help

 

If the truck has A/C , don't forget to check the air conditioner compressor pulley bearing.
Sometimes if the pulleys are not all lined up it can cause the belt to wear faster and squeal on start-up.

 

A worn belt can also cause the same sound. Grips better as it warms up, but still slips a little. I change my belt(s) every 4 years/50K miles. I also keep the one that I just changed out as a traveling spare...

 

Welcome aboard..

How many miles on the truck?
Belt is new?

Sometimes the tensioner spring/assy will 'hang' and not apply full tension on the belt. If the belt is pulled back beyond the normal run area of the tensioner, and released, the tensioner should snap back.
When the idlers were changed did you check tensioner travel?

Like UCS mentioned, a used/dried belt can squeal/chirp more so cold, then diminish or run silent heated. An aged belt issue can usually be determined with a short shot of WD (disipates quickly) on the belt running surface. No oil based sprays/lubes. If it instantly quiets, it rules out bearings, but not always spec. belt tension. A mechanics stethoscope can help pinpoint hub/idler bearings, or with the belt removed and rotating each possible source for smooth operation.